1. Field of the Invention
The present invention relates to a semiconductor substrate, a semiconductor device and a method of manufacturing the semiconductor device, and, more particularly, to a semiconductor substrate having a nitride semiconductor layer, a semiconductor device using the semiconductor substrate and a method of manufacturing the semiconductor device.
2. Description of the Related Art
The nitride semiconductor layer is typically formed on an inexpensive silicon substrate or a sapphire substrate. However, these substrates have lattice constants largely different from a lattice constant of the nitride semiconductor layer, and have coefficients of thermal expansion different from a coefficient of thermal expansion of the nitride semiconductor layer. Therefore, large distortion energy is generated in the nitride semiconductor layer formed on the substrate by epitaxial growth. As a result, a crack is likely to occur on the nitride semiconductor layer or crystal quality is likely to degrade.
To solve the above-described problem, conventionally, a buffer layer in which nitride semiconductor layers having different composition are stacked has been disposed between the substrate and a functional layer comprised of a nitride semiconductor.
Further, to improve characteristics of the nitride semiconductor layer, various proposals have been made as to the buffer layer.
For example, Patent Literature 1 discloses that the buffer layer contains boron to suppress dislocation.
Further, Patent Literature 2 discloses using a nitride aluminum layer which contains boron whose concentration increases toward the functional layer as the buffer layer to obtain a boron containing nitride aluminum thin film (functional layer) having an excellent crystallinity.
Still further, Patent Literature 3 discloses suppressing a leak current due to two dimensional electron gas in the buffer layer by using the buffer layer including a first layer comprised of a GaN layer containing boron and phosphorus and a second layer comprised of an AlInGaAsN layer containing phosphorus.